Blade wheels in radial flow gas turbines are for convenience in manufacture often made in two sections which are assembled by being clamped together endwise. The clamping may for instance be effected by connecting means including a bolt or sleeve secured to and protruding from the end face of the hub of one section, which may be a turbine wheel section, and extending through a central bore in the hub of the other section, which may be an educer wheel section. The axial clamping or connecting means may also include a resilient compression sleeve surrounding the connecting bolt or sleeve, one end of said compression sleeve engaging a first shoulder in the bore of the educer wheel section and the other end engaging a second shoulder at the end of the connecting bolt or sleeve. Such as assembly is disclosed in U.S. Pat. No. 3,628,886 and GB Pat. specification No. 1,292,858.
When the gas turbine is put into operation the sections and the connecting means will be heated and expand in the axial direction. When the turbine has been in operation for some time a steady state is reached in which the connecting bolt or sleeve is usually somewhat hotter than the hub of the educed wheel section. The difference in temperature T.sub.conn -T.sub.hub depends upon the load on the turbine and attains its maximum value at full load. This difference in temperature is due to the fact that the connecting bolt or sleeve is directly connected to the first or turbine wheel section with which the hot combustion gases are first contacted, and which, therefore, will be hotter than the hub of the educer wheel section. However, when the cold turbine is put into operation the educer wheel section will during a transient period be hotter than the elements of the connecting means directly connected to the turbine wheel section. Thus, T.sub.conn -T.sub.hub &lt; 0. Conversely, the temperature T.sub.hub in the educer wheel section will drop more rapidly than the temperature T.sub.conn in these connecting elements when the load on the turbine is reduced from full load to zero load. In a transient period the difference in temperature between the connecting elements and the hub of the educer wheel (T.sub.conn -T.sub.hub) will therefore be still larger than in the steady state at full load.
Because of heat elongation these variations in temperature entail a large variation in the axial clamping force provided between the two sections by the connecting means. Thus, substantial problems are encountered with respect to the axial clamping. On one hand the clamping force should not fall below a certain minimum value. If, on the other hand, the elongation of the hub of the educer wheel section resulting from the heat expansion is to its full extent imposed on the connecting sleeve, stresses far exceeding the yield point are created in the connecting sleeve, thereby producing a permanent elongation of the connecting sleeve and a loosening of the clamping when the connecting sleeve again becomes hotter than the hub (T.sub.conn -T.sub.hub &gt; 0).
To avoid this, the U.S. and G.B. patent specifications referred to teach that as little as possible of the elongation of the hub of the educer wheel should be transmitted to the connecting sleeve. This is achieved by so positioning said first shoulder within the bore that said shoulder is spaced only a short axial distance from the radial plane of the torque transmitting engagement area, and dimensioning said resilient compression sleeve to have such clearances relative to the bore of the hub and the connecting sleeve, respectively, that the compression sleeve will obtain a temperature much closer to the temperature of the connecting sleeve than to the temperature of the hub. In this manner the heat elongation which is imposed on the connecting sleeve when the negative difference in temperature between the connecting sleeve and the hub attains its maximum absolute value is substantially reduced, since only a small portion of the heat elongation of the hub is transmitted to the connecting sleeve, the compression sleeve at the same time serving as a resilient member relieving the connecting sleeve.
However, in connection with the development of large radial flow gas turbines having higher temperatures it has proved that the transient temperature differences require resilient members having a substantially larger resiliency than what can be obtained by such a compression sleeve. It is true that the resiliency of the clamping may be increased by using a series of co-axial sleeves mutually engaging each other at the ends, for instance an inner and an outer compression sleeve and an intermediate tension sleeve, but since the size and the operating conditions of radial flow gas turbines presently under development may require the connecting means to accommodate a variation in the axial dimension of as much as 5 to 10 times the elastic compression which may be obtained by an ordinary compression sleeve, said solution using several co-axial sleeves is not practicable, as it would require more space than available.